U.S. antitrust law is complicated, but current efforts to block a merger between grocery retailers Albertsons and Kroger may not fit the bill. Current triggers under the law include:

Market share of 70 percent or more, or less than 50 percent if barriers limit competition. Barriers to entry prevent competition. When a firm can raise prices above competitive levels without losing market share.   The use of predatory practices. A Herfindahl-Hirschman Index score of 2,500 or higher, or possibly 1,500 to 2,500.

The proposed Albertson/Kroger merger would result in a firm still smaller than Walmart. Under the Rule of Three theory, firms consolidate until three firms dominate a market. GM, Ford and Chrysler once comprised the B...